1. Field of the Invention
The present invention pertains to head flowmeters for measuring the air or gas flow rates through conduits, especially at low flow rates, and more particularly, it pertains to head flowmeters of the type which attempt to minimize the generated turbulence in the flow measurement area by presenting a restriction with a plurality of closely spaced orifices to the air flow and measuring the pressure drop thereacross in order to determine the flow rate.
2. Description of the Prior Art
In one type of conventional flowmeter, sometimes referred to as a head flowmeter, particularly for measuring the flow of air or other gases, a device designed to introduce a slight resistance to the flow of the air is placed within a conduit through which the air is flowing. As is well-known in head flowmeters, the flow rate can be directly measured within a pre-calibrated range in a given conduit and at a given temperature and pressure by measuring the pressure drop across a restrictive orifice or plurality of orifices since flow is proportional to the square root of the differential pressure across the restriction. Thus, the difference between the pressures appearing at the wall of the conduit sections just upstream and downstream of the flow impeding device is measured, and the magnitude of that pressure difference is directly utilized to determine the flow rate of the air (in standard cubic feet per hour, SCFH, for example) flowing through the conduit (i.e., the greater the pressure differential, the greater the flow). In its simplest form, the flow impeding device may be formed by a thin plate mounted perpendicularly to the axis of the air flow conduit in such a manner so as to force the entire flow of air through an orifice machined in the plate and located on the centerline of the conduit. In order to reduce the turbulence at the flow impedance device, which can result in inconsistent and erroneous readings in the output of the flowmeter, a plurality of generally uniformly distributed orifices may be provided in the orifice plate, which thus may come to resemble a screen. In fact, many prior art arrangements have used a conventional woven screens for the flow impeding devices with pressure taps being provided just upstream and downstream of the screen to monitor the differential pressure and hence the flow rate.
Prior art air flow measuring devices of the foregoing type which have used typical screens of the woven mesh type to provide the plurality of restricted orifices across the air flow path are shown, for example, in U.S. Pat. No. 3,504,542 to Blevins, U.S. Pat. No. 3,626,755 to Rudolph, U.S. Pat. No. 3,797,479 to Graham and U.S. Pat. No. 5,357,972 to Norlien. With woven mesh type screens, however, an air flow problem on the upstream side of the screen is presented since the air flow (particularly at low flow rates) tends to follow the undulations of the weave at the mesh openings. This results in a tendency to create a circular air pattern about each of the openings in the mesh which vary in accordance with the air flow velocities and thereby hinder accurate readings of flow rate over a suitable range.
Other prior art flowmeter devices of the type described have provided elongated tubular configurations (such as those shown in U.S. Pat. No. 3,071,160 to Weichbrod and U.S. Pat. No. 3,838,598 to Tompkins) for the flow restriction, which for various reasons have not proven to be wholly satisfactory.
Finally, flat plate restrictors have been used with multiple spaced orifices therein such as shown, for example, in U.S. Pat. No. 5,722,417 to Garbe and U.S. Pat. No. 3,129,587 to Hallanger. Such devices have not proven to be easily manufacturable and readily adaptable for use in various environments, such as in the measuring of relatively low air flow in underground conduits for the pressurization of telephone cables or the like.